The voiced aspirated stops may have first become voiced fricatives before hardening to the voiced unaspirated stops "b", "d", and "g" under certain conditions, however some linguists dispute this. See Proto-Germanic phonology.

This is strikingly regular. Each phase involves one single change which applies equally to the labials (p, b, bʰ, f) and their equivalent dentals (t, d, dʰ, þ), velars (k, g, gʰ, h) and rounded velars (kʷ, gʷ, gʷʰ, hw). The first phase left the phoneme repertoire of the language without voiceless stops, the second phase filled this gap but created a new one, and so on until the chain had run its course.

Note: Icelandic hv has actually reverted Grimm's Law in the last few generations, and is now pronounced [kʰv] or [kʰf]. Cf. also nynorsk kv-/k-.

Exceptions

The voiceless stops did not become fricatives if they were preceded by *s (itself a fricative).

English: short, Old Norse and Icelandic: skorta, Old High German: scurz

Lithuanian: skurdus

*skʷ

English: scold, Old Norse: skäld, Icelandic: skáld, Dutch: schelden

Irish: scioll

Note: Some linguists dispute the origin of the word "scold", but Julius Pokorny among others proposed *skwetlo is the assumed root.

Furthermore, the voiceless stop *t also did not become a fricative if preceded by *p, *k, or *kʷ (themselves voiceless stops). The voiceless stop it was preceded by did fricativize, however. (In other words, at the time in history when voiceless stops fricativized in Proto-Germanic, that fricativization only affected leading voiceless stops when paired with the voiceless stop *t.) This is sometimes treated separately under the heading Germanic spirant law:

The most recalcitrant set of apparent exceptions to Grimm's Law, which defied linguists for a few decades, eventually received explanation from the Danish linguist Karl Verner (see the article on Verner's law for details).

Correspondences to PIE

The Germanic "sound laws", combined with regular changes reconstructed for other Indo-European languages, allow one to define the expected sound correspondences between different branches of the family. For example, Germanic (word-initial) *b- corresponds regularly to Latin *f-, Greek pʰ-, Sanskritbʰ-, Slavic, Baltic or Celticb-, etc., while Germanic *f- corresponds to Latin, Greek, Sanskrit, Slavic and Baltic p- and to zero (no initial consonant) in Celtic. The former set goes back to PIE *bʰ- (faithfully reflected in Sanskrit and modified in various ways elsewhere), and the latter set to PIE *p- (shifted in Germanic, lost in Celtic, but preserved in the other groups mentioned here).